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New catalytic converter - blowing black smoke then enging shutdown

My car (2007 Mustang v6 - 156k miles) didn’t pass inspection due to a check engine light on. Long story short, catalytic converter needed replacement. However, after getting my car back from the mechanic, I noticed the car’s engine sounding rougher than before when driving it home about 30 minutes from the shop. The next day, I planned to take it to state inspection but I stopped by the store first. Then I couldn’t start anymore. It tried to crank up but it wouldn’t start. Black smoke was blowing out of the exhaust and it smelled fuel all over.

I had it towed to the shop and I was told it was the spark plug and the crank sensors. Had those replaced and car still does not work. Now they’re telling me it’s the pistons and the motor needs lots of work. I’m starting to doubt the shop I brought it to. They did a full diagnostic before changing the catalytic converter and they told me the motor is fine but I needed new brakes and differential gasket. But that was it.

I would appreciate any opinions on what I should do next. SHould I rebuild engine? Changed shop? Did they do anything to the catalytic converter that cause engine shutdown?

Go to a different shop now!

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Catylitic converter swap can not ruin engine, but a bad engine can ruin the original converter. Because they did an engine diagnosis and missed the bad pistons, you need to find a better shop. Can’t recommend repair or replace because we don’t know year, make, model or milage.

Thank you. It’s a 2007 Mustang v6 - 156k miles. Do you know why black smoke was blowing after they placed the new catalytic converter and why it smelled like fuel everywhere. I am female and I don’t know anymore if I am being fooled by these folks.

Is your check engine light on? With black billowing smoke, it should be. If so, find out what codes are stored…

There are many possible causes of running too rich. The O2 sensors are located before and after the catalytic converter(s). If the forward sensor(s) are disconnected or defective, they can result in the engine running rich. Lots of other reasons as well…

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How was the car running before the cat was replaced? A bad engine/pistons is NOT going to run right. So if it was running OK, then I seriously doubt it can be a bad engine/pistons. Find a different mechanic. I don’t think this guy knows what he’s doing.

The engine was running fine. There was no weird sound whatsoever. I always listen to my car for the first few minutes every time I get in it before turning my radio on. There was absolutely no weird sound. The car was accelerating fine. I even drove it on the freeway for about 100 miles round trip and it was fine.

I just don’t know if the car can suddenly just go bad like that after running fine the days before.

The check engine light is not on. It was on before catalytic converter was changed - and the code was catalytic converter - according to autozone.

So i took it to the shop and paid for diagnostic. But when I ask for the results of the diagnositc - they just said catalytic converter and couldn’t show me anymore analysis. not even paperwork. It looks like I was charged $99 diagnostic fee for them to stick that same tool autozone used to tell me it’s catalytic converter instead of analyzing whether it’s cat or O2 sensors etc. Now, I don’t know about cars at all.

After they replaced cat,( they said they used OEM - the entire thing - not the one that needs to be welded) that’s when all these problems began - black smoke in exhaust, fuel smell.

Lemme 2nd and 3rd the comments. Take the car to a different shop. These twits are clueless.

If you can drive it, either go to another shop or back to autozone to have them scan it again and check for any pending codes. I’d be careful about too much driving, you could ruin the new cat by running it so overly rich for too long…

There is no code that definitively says it’s the Catalytic Converter. The O2 sensors give out bad reading which points to the Cat…HOWEVER…further testing needs to be done to determine if it’s an O2 sensor problem or the Cat. Any mechanic who determines you need a new Cat bases solely on the scan reading is a lousy mechanic.


Thank you. I just called the shop again and I was told it’s jump timing. No mention of the pistons again. I am getting the car towed to another shop tomorrow.

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Smart move.

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Make sure you document everything…you actually haven’t taken the car to get re-inspected yet, after all. Especially if it turns out that the catalytic converter wasn’t the issue (along with the host of other items), it seems like you may have a case against this shop in small claims court.

Not much chance this is the problem, but it could be, and the test is very easy to do. Ask your shop to make sure the brake booster is holding vacuum to 20 inches.

The Autozones around us only use a basic scanner that reads active codes. They don’t use one that can read pending. (this of course could be different in different areas of the country)